Kelly O'Brien, whose fiancee was murdered in the Tucson shooting, appeared on Good Morning America to make the case that regulating high-capacity magazines made sense. O'Brien stated:

I absolutely support it...And it is so sad to see 19 people gunned down in just 15 seconds by one of these extended capacity clips

[...]

As you know Jared was stopped when he ran out of bullets. And I believe, even if not Gabe, other people could have been saved that day. And absolutely more people could have not had the injury and stuff that they had to sustain.

Even Robert A. Levy, who was co-counsel in the landmark legal case that established the Second Amendment as protecting an individual's right to bear arms, acknowledges that regulating high-capacity magazines would be constitutional. As reported by NBC:

Robert A. Levy, who served as co-counsel in the landmark Supreme Court case that established a Second Amendment right to bear arms, said there was no reason the court's decision in that case should apply to the purchase of high-capacity gun magazines.

"I don't see any constitutional bar to regulating high-capacity magazines," Levy said in an interview with NBC. "Justice [Antonin] Scalia made it quite clear some regulations are permitted. The Second Amendment is not absolute."